Various systems can be used to actively control the timing and lift of engine valves to achieve improvements in engine performance, fuel economy, emissions and other characteristics. One class of systems, called lost-motion system, has been disclosed in U.S. Pat. Nos. 4,671,221, 5,193,494, 5,839,400, 6,053,136, 6,553,950, 6,918,364, 6,981,476, 7,819,100, 8,578,901, 8,820,276 and 8,776,738. Each of the above lost-motion systems invariably comprises a cam, a master piston, a high pressure chamber, a mechanism to drive an engine valve, an on/off or 2-way solenoid valve as a release valve or trigger valve, an engine valve and an engine-valve return spring. Most of these systems, as disclosed in U.S. Pat. Nos. 4,671,221, 5,193,494, 5,839,400, 6,053,136, 6,553,950, 6,918,364, 6,981,476, 7,819,100, and 8,578,901 further includes at least one slave piston, which is separated from the master piston by a volume or column of fluid in the high pressure chamber and is operably connected the engine valve. A second slave piston may also be added to drive a second engine valve. The cam drives the engine valve, in the opening direction, through an optional tappet, the master piston, high pressure chamber and the slave piston. The engine-valve return spring returns the engine valve in the closing direction. The cam profile normally defines the valve displacement or lift profile or lift. The solenoid valve is typically a normally-open valve and has to be switched to the closed state for the high pressure chamber to maintain a high pressure and transfer the cam lobe into the engine valve displacement. When the solenoid valve is not closed early enough, the engine valve will be opened late and lose some of the total available valve movement or motion. When the solenoid valve is opened before the end of the cam lobe, a fast bleeding of the fluid from the high pressure chamber will cause the engine valve to collapse and close early, losing some motion or lift.
Other systems, disclosed in U.S. Pat. Nos. 4,8,820,276 and 8,776,738, further comprises a mechanical motion transfer system, which further includes a rocker arm, a pivoting axle, a push rod, a level pushrod, and a pivoting bridge. The cam drives the engine valve operably through the mechanical motion transfer system and with the master piston as an adjustable reference point or foundation. With the master piston at its fully extended position, the engine valve realizes its full lift profile. When the solenoid valve either closes too late or open too early, the engine valve will lose some of the potential motion.
When a control signal 19 is issued as shown in FIG. 1, the actual plunger or spool displacement 20 of a solenoid valve does not following the signal exactly. The control signal 19 in FIG. 1 is idealized for easy presentation, and the actual input current to a solenoid has more content or dynamics, for example, a peak-and-hold pattern for faster response and low energy consumption. For a normally-open valve, it is closed effectively and fully when it is energized, with its displacement 20 reaching its closure threshold Xo and the maximum Xmax, respectively. For a valve of a poppet design, a plunger has to be fully seated to have an effective closure, i.e., Xo=Xmax. For a valve of a sliding spool design, an overlap is normally included to reduce leakage, thus Xo<Xmax. The valve achieves an effective closure with a close time delay T1. Under the action of a control-valve return spring, the valve opens up again after the solenoid is off, with an open time delay T2. The time delays T1 and T2 result from solenoid inductance, inertia of the all moving masses, viscous force, hydraulic pressure force, etc. For a typical solenoid valve for a lost-motion system, the solenoid inductance is generally a dominant factor under normal (i.e., warm and hot) temperature conditions. The viscous force plays increasingly a more important role at a colder temperature, even a dominant role under an extremely low temperature. The solenoid inductance is also impacted by the coil temperature. The viscous force also varies with the clearance, wear and dynamic eccentricity between two moving parts. Hydraulic pressure force varies with the system conditions, and more so at low temperatures.
For a typical solenoid valve for a lost-motion system, T1 and T2 are typically around a few milliseconds or shorter depending on the design trade-off, and they are generally required to be short for better control flexibility. At an extremely cold temperature, e.g. under −20 deg C. T1 and T2 may be longer than 10 milliseconds.
For an engine cam system, manufacturers typically require that engine valve profiles have an accuracy within +/−1 crank angle degrees, that is within +/−0.167 ms and +/−0.0556 ms in timing for the engine speed at 2,000 rpm and 6,000 rpm, respectively. Therefore, an accurate control or prediction of time delays T1 and T2 are important. In addition, the closure threshold Xo is in a way only one theoretical or artificial point, the flow restriction is present throughout its vicinity. The shape, i.e. curvature and slope, of the displacement 20 around Xo is also important. It is desirable to consistently control and/or predict the displacement 20.
The lost-motion valve systems, branded as MultiAir and UniAir, are now in production in Fiat and Chrysler vehicles, and their key advantage over variable valve timing system is their ability to control intake air volume without use of a throttle body, thus reducing air pump-loss and achieving 5% or more incremental fuel economy benefits. Application vehicles, however, still use the throttle body for air intake control during engine warm up because of above discussed low-temperature operation challenges. Any improvement dealing with these challenges will also help broaden application of the lost-motion systems to other advanced combustion modes, such as HCCI (homogenous charge compression ignition).
Accordingly, there remains a need for a method and apparatus for accurate and robust control of the trigger valve.